1. Field of the Invention
The present invention relates to a method of and an apparatus for assembling units in an assembly line in which component parts, including a tentative component part, are provisionally assembled into a unit, in which, after an inspection as to a positional relationship among the components, the unit is disassembled for replacement of the tentative component part with another as a permanent component part, which is suitable to provide a predetermined positional relationship among the component parts, and in which the disassembled unit is assembled again as a complete unit.
2. Description of Related Art
In the course of recent technological advancement, it has proved to be necessary to increase, as far as possible, the precision of assembling component parts into units such as industrial products, for instance automobiles, industrial machines, etc., in so-called automated assembly lines to achieve a high accuracy and a uniform quality, so that the complete products exhibit a high performance.
For instance, engine cylinder heads have various component parts installed around cylinders such as, for instance, intake and exhaust valves, a camshaft, etc., which have repercussions of great importance on good engine performance such as, in particular, speed and fuel efficiency of an engine. It is known that a relative position between a valve and a cam is of particularly great importance in terms of the sealing performance of the valve, the impact of the closing valve and the lift of the valve. Consequently, it is necessary to inspect the relative position of the valve relative to the cam after having assembled them to the cylinder head and, if necessary, disassemble the cylinder head for the replacement of one specific component part with another so as to provide a desired relative position.
For the purpose of enhancing an understanding of the work of reassembling, reference is made to FIG. 1.
FIG. 1 shows part of a cylinder head 1 of an overhead camshaft engine for an automobile. After having installed a valve 2 and a tappet 3 to the cylinder head 1, an overhead camshaft 6 is placed between a semi-circular metal bearing 5, formed integrally on the cylinder head 1, and a semi-circular metal bearing 8, formed as a bearing cap 7 and held stationary therebetween by fastening nuts 10 onto bolts 9. If the overhead camshaft 6 has been assembled to the cylinder head 1 with its cam 11 placed too closely to the tappet 3, then the valve head of the valve 2 tends to be seated loosely on a valve seat 12. On the other hand, if the overhead camshaft 6 has been assembled to the cylinder head 1 with its cam 11 placed improperly far from the tappet 3, the valve head of the valve 2 tends to cause an impact against the valve seat 12 during closing due to a steep angle at which the cam 11 is brought into contact with the tappet 3, resulting in a shortened useful life of the cam 11. In order for the camshaft 6 to provide an appropriate valve clearance between the cam 11 and tappet 3, it is necessary to inspect or examine whether or not the valve clearance is proper after the camshaft 6 has been assembled to the cylinder head 1 and, if the valve clearance is improper, replace some parts for valve clearance adjustment. This parts replacement demands disassembly and re-assembly of the cylinder head 1. In order for the adjustment of valve clearance to be executed easily, it has typically been known to place a shim 4 in a top recess 3' of the tappet 3. One shim 4, which is a thin disk, is replaced with another having a thickness suitable to provide a desired valve clearance.
Much attention has been given to various approaches relating to valve clearance inspection techniques. One such approach is that described in, for instance, Japanese Unexamined Utility Model Publication No. 63-156009.
If the semi-circular metal bearings 5 and 8, fitted to each other, do not form a true round bearing bore, it is difficult to insure alignment of the camshaft 6 with the bearings 5 and 8, and consequently maintain a desired valve clearance. In order to eliminate any irregular alignment, it is necessary to perform boring between the cylinder head 1 and the bearing cap 7 previously secured to the cylinder head 1. In the course of installing or assembling component parts of a valve mechanism such as, for instance, the valve 2, the tappet 3 and a standard shim disk 4 to the cylinder head 1 with a camshaft bearing bore formed in that manner, the parts can be assembled only after the removal of the bearing cap 7 from the cylinder head 1. Thereafter, the camshaft 6 is assembled and held for rotation by the metal bearings 5 and 8. After an inspection of valve clearance, if the standard shim disk 4 is improper in thickness, it must be replaced with another shim disk suitable to provide a desired valve clearance. It is necessary, for this shim disk replacement, to perform disassemblage and reassemblage of the cylinder head 1 once again. Accordingly, disassembling and reassembling the bearing cap 7 and the camshaft 1 has to be repeated two times, once before and another after the inspection of valve clearance, until the cylinder head 1 is complete.
Installation of disassembling and assembling stations in series, both before and after the inspection station, makes a production line too long, leading not only to inefficient use of space in a factory but also to a large investment in installation of control units and drive systems for the respective work stations in order to rule over the production line, including the work stations, altogether. This results in an increase in production costs.
If there are provided transportation conveyers between adjacent work stations of a series arrangement of a plurality of different work stations and these work stations take different working cycles, then there unavoidably occurs an accumulation of units to be in front of before any work station which takes a working time longer than the preceding work stations. In other words, the units must be left to wait for a long time at the preceding station, causing a reduction in productivity of the production line.